Luc Salvo

X-ray micro-tomography an attractive characterisation technique in materials science

X-ray tomography is a non-destructive technique which provides 3D information of materials. It is consequently very attractive in Materials Science since the relation between macroscopic properties and the micro-structure of a material is very frequently required. The aim of this paper is to present selected results obtained in various investigations of metallic materials such as superplastic deformation, materials in the semi-solid state and metallic foams. Depending on the studied features, several tomography analysis modes were used: conventional absorption mode, phase contrast and holotomography, a new technique, which provides the 3D distribution of the electron density in the bulk of the material. Furthermore micro-tomography enables one to perform in situ experiments either by using a mechanical test machine or a furnace.

Quantitative phase contrast tomography using coherent synchrotron radiation

The coherence of third generation synchrotron beams makes a trivial form of phase-contrast imaging possible. It is based on propagation and corresponds to the defocusing technique of electron microscopy. The propagation technique can be used either in a qualitative way, mainly useful for edge- detection, or in a quantitative way, involving numerical retrieval of the phase from images recorded at different distances (typically three or four) from the sample. The combination with tomography allows to reconstruct the electron density in the sample with micrometer resolution. This combined approach is called holotomography. It was applied to several problems in materials and life sciences when it is crucial to enhance the sensitivity or reduce the dose compared to absorption tomography. Pure phase objects such as foams and fleece structures can be imaged with excellent contrast and resolution. Holotomography turned out to be a invaluable tool to study semi-solid materials with two metallurgical phases that have similar attenuation coefficients. The attenuation and density map yield in this case complementary information, the latter being the useful one to study the connectivity of the solid phase. The dose reduction and increased sensitivity in phase imaging are crucial for imaging thick (millimeter range) biological samples in their natural, wet environment. Results obtained on Arabidopsis plant indicate the possibility to investigate at the micron scale the spatial organisation of plant cells.